Seattle used to have this for an area of the city. It was called the "Free Ride Zone" - and it had some unexpected consequences. The primary problem is that homeless people would get on the bus because it's warm, and would fall asleep. The busses became rolling homeless shelters. This caused other people to take the bus less, because of public safety concerns, as well as simple quality-of-life issues like unpleasant…
Interestingly, here in Melbourne we have a similar system when it comes to paying fares. It's not the bus/train/tram drivers responsibility to ensure you pay your fare, you can walk onto a bus without tagging on and the driver won't even blink, it's the job of roaming ticket inspectors to enforce fare payment. In my experience, we don't have a particularly significant problem with homeless people and other "undesirab…
I have a feeling the people complaining about the homeless would say the same things encountering "undesirables" on our PT though, it's just a disguised "I have to interact with poor people" argument.